This invention pertains to electromagnetic discharge devices and, more particularly, is concerned with ultraviolet light sources.
Perhaps the most familiar electromagnetic discharge ultraviolet source is the common fluorescent lamp. Usually the lamp has a cylindrical envelope filled with low pressure inert gas (e.g., argon) and a small dose of metallic mercury. Voltage applied to electrodes within the envelope accelerates electrons which ionize the inert gas, initiating a discharge. Heat and electrons from the discharge vaporize and excite the mercury which emits ultraviolet and visible radiation, with a strong ultraviolet line at 253.7 nm. A phosphor layer inside the envelope converts the ultraviolet to visible light.
Many modifications have been proposed to improve the conventional fluorescent lamp. Departing from a straight tube configuration, envelopes have been formed into toroids, spheriods, re-entrant cavities, and many other configurations. Beam shaping electrodes have been demonstrated, as have electrodeless discharges. Most of these modifications, however, call for mercury in the discharge medium.
Effort has also been made to improve the filling. For examples, U.S. Pat. No. 4,427,921 issued January 24, 1984 to Proud et al for "Electrodeless Ultraviolet Light Source" discloses fillings including I, HgI.sub.2, and CdI.sub.2, and U.S. Pat. No. 4,427,922 to Proud et al for "Electrodeless Light Source" describes fillings including HgI.sub.2, HgBr.sub.2, and HgCl.sub.2.
In the related art of high pressure mercury vapor lamps it has been known for a number of years to improve the visible output of such lamps by adding metal halides to a filling of mercury and inert gas. U.S. Pat. No. 3,586,898 "Aluminum Chloride Discharge Lamp" issued to Speros and Simper divulges a filling of aluminum trichloride, mercury, and inert gas with the optional addition of aluminum tri-iodide. The lamp envelope is either alumina or alumina coated quartz to avoid reaction between AlCl.sub.3 and SiO.sub.2.
Mercury and cadmium are known to accumulate in biological systems and are hazards to human health. While the dosage of these metals expected from individual lamps is likely to be below the threshold of harm, it would be desirable to avoid their use if an alternative efficient fill material were available.
Accordingly, it is an object of this invention to provide an efficient discharge ultraviolet light source having fillings free of mercury or cadmium. Another object is to provide an ultraviolet lamp source having greater radiant intensity than a mercury lamp of the same physical size.